My wife and I take at least one diving vacation every summer, usually around the middle of June when typically the weather here is just starting to warm up and dry out. I don't like to go anywhere in the winter because it means having to return to the wet and cold of the Pacific Northwest, and I'd rather just gut it out. July through September is by far the best time here weather-wise, so I'd rather hang out at home and enjoy it during that time.
I'm the planner in the family. My wife trusts me to pick places we'll both enjoy, and I like doing it. Dive magazines and advice from my LDS help a lot along with participating here on Scubaboard and surfing the world wide web.
Typically, I start planning for the next trip within a week or two of returning from the last trip. Often, I get ideas from where we went that summer. We started with Playa del Carmen/Cozumel because that's where everybody goes. Next was Belize because we were just so close when we were in Mexico. Hawaii and then the Florida Keys just sort of fell into our lap. Back to Belize because there was some unfinished business on the mainland before diving. PdC/Cozumel a couple of more times thrown in there since I can't seem to get enough of the cenotes. Next year it's southern Belize in hopes of diving with the Whale Sharks.
I'm guessing we'll head further south after that, but I probably won't know for sure until we do the Whale Shark thing.